Low power mode and feedback arrangement for a switching power converter

ABSTRACT

A low power mode and feedback arrangement for a switching power converter. Two or more main power switches, such as transistors, transfer energy from a supply to load by their opening and closing. When the load requires a relatively low power level, this condition is detected. In response, one or more of the transistor switches is disabled from switching and the reduced power requirements of the load are handled by the remaining one or more transistor switches. As a result, switching losses are reduced. This is because parasitic gate capacitance and on-resistance associated with the disabled switches no longer consume power from the power source. The invention provides significant efficiency advantages during periods when the load draws a low level of power. This is especially useful for battery-powered devices which may operate in a low power mode for extended periods of time, such as standby mode as in a portable telephone. This is because the prevention of power loss in may contribute considerably to operating time before battery re-charging is needed. However, when additional power is required, one or more previously disabled switches may be brought back into play to ensure that the power requirements of the load are met. In an audio amplifier having a H-bridge arrangement for providing a differential output signal, the invention also provides a feedback path from only one side of the load. This reduces distortion associated with open-ended operation.

BACKGROUND OF THE INVENTION

The invention relates to the field of switching power converters and amplifiers. More particularly, the invention relates to switching power converters and amplifiers having a low power mode for conserving power.

In a conventional switching-mode power converter, a current from a power source, such as an unregulated supply, passes through an inductor when a main power switch is closed. This charges the inductor with energy. When the switch is opened, the energy is discharged into a capacitor. This forms an output voltage across the capacitor which may then be used for driving a load. This output voltage is maintained at a constant, desired level by appropriately controlling the opening and closing of the switch, such as by pulse-width modulation (PWM) or frequency modulation.

In accordance with PWM, the duty-cycle for opening and closing the main power switch is controlled. For example, a periodic ramp signal is compared to a variable signal to control the duty cycle of the switch. A level of power delivered to the load depends upon a duty cycle of the main power switch.

In accordance with frequency modulation, the frequency at which the main power switch is opened and closed is controlled. For example, a voltage-controlled oscillator (VCO) can be utilized for controlling the main power switch. A level of power delivered to the load depends upon the switching frequency.

A conventional switching power amplifier is a type of switching power converter in which the output is varied in response to an input signal. This is in contrast to a switching power converter in which the output is typically maintained at a constant level. In a typical switching power amplifier, the opening and closing of the main power switch is controlled in response to a level of the input signal. Therefore, the output of the switching amplifier tends to follow the input signal.

A class D audio amplifier is an example of a conventional type of switching power amplifier which provides a differential output signal across a speaker using an H-bridge arrangement of four main power switches. Each pair of the switches is coupled serially between a high potential of a power supply and a low potential. Nodes intermediate to each pair of power switches are connected to opposite terminals of the speaker. By controlling the opening and closing of the four main power switches in accordance with an input audio signal, the audio speaker generates sounds related to the input signal. Because the output signal is differential, such an amplifier is typically operated in open loop (i.e. without feedback). A drawback to open loop operation of such an amplifier is that the output signal is susceptible to distortion.

Conventional switching power converters tend to provide efficiency advantages over other types of devices. This is because switching power converters tend to draw power at a rate commensurate with the requirements of the load and tend to have relatively low energy losses. Energy loss in a switching power converter is typically referred to a switching loss and is largely due to energy dissipation in the main power switch(es) and in the reactive elements, such as inductors or capacitors. When a load consumes a level of power which is near the maximum capacity of the switching power converter, the switching losses are low in comparison to the total power drawn by the switching power converter. Thus, the converter operates with high efficiency. As the level of power consumed by the load decreases, however, switching losses become more significant in comparison to the level of power drawn from the power supply. Thus, at low levels of load power consumption, switching losses can significantly reduce efficiency. Efficiency, however, is often an important performance criteria for a switching power converter. For example, where the power is drawn from a battery supply, such as in a portable telephone, low efficiency results in reduced periods of operation between battery charges. Therefore, conventional switching power converters tend make inefficient use of battery power when a load draws a low level of power.

Therefore, what is needed is a switching power converter which does not suffer from the aforementioned drawbacks. It is to these ends that the present invention is directed.

SUMMARY OF THE INVENTION

The present invention is a low power mode and feedback arrangement for a switching power converter. Two or more main power switches, such as transistors, transfer energy from a supply to a load by their opening and closing. When the load requires a relatively low power level, this condition is detected. In response, one or more of the transistor switches is disabled from switching and the reduced power requirements of the load are handled by the remaining one or more transistor switches. As a result, switching losses are reduced. This is because parasitic gate capacitance and on-resistance associated with the disabled switches no longer consume power from the power source. The invention provides significant efficiency advantages during periods when the load draws a low level of power. This is especially useful for battery-powered devices which may operate in a low power mode for extended periods of time, such as standby mode as in a portable telephone. The prevention of power loss may contribute considerably to operating time before battery re-charging is needed. However, when additional power is required, one or more previously disabled switches may be brought back into operation to ensure that the power requirements of the load are met.

In an audio amplifier having a H-bridge arrangement for providing a differential output signal, the invention also provides a feedback path from only one side of the load. This reduces distortion associated with open-ended operation.

In accordance with one aspect of the present invention, a switching power converter is provided having first and second transistor switches. Energy is transferred from a supply to a load by opening and closing the first transistor switch; energy is transferred from the supply to the load by opening and closing the second transistor switch; and the second transistor is disabled from switching upon detection of a low power condition.

The first switch may transfer energy from the supply to a first reactive element upon closing and the first switch may transfer energy from the first reactive element to a second reactive element upon opening. The second transistor switch may transfer energy from the supply to a third reactive element upon closing and the second transistor switch may transfer energy from the third reactive element to the second reactive element upon opening. Switching of the first and second transistor switches may be interleaved. The first and second transistor switches may be coupled in parallel to each other. Switching of the first transistor switch may be synchronized with switching of the second transistor switch when the second transistor switch is actively switching. The first transistor switch may be disabled from switching when the second transistor switch is actively switching. The switching power converter may include a pulse-width modulation switch controller coupled to the first and second switches. The switching power converter may include a frequency modulation switch controller coupled to the first and second switches. The low power detector may detect the low power condition by monitoring a voltage provided to the load or by monitoring an error signal representative of a difference between a voltage provided to the load and a desired level for the voltage. The low power detector may include a comparator having a hysteretic transfer characteristic. The first and second transistor switches may have substantially different current-carrying capacities. Energy may be transferred to the load by a current which is synchronously rectified. The switching power converter may also include a third transistor switch and a fourth transistor switch, the third and fourth transistor switches for performing synchronous rectification on the current, wherein the fourth transistor is disabled by the low power detector upon detection of the low power condition.

In accordance with a further aspect of the invention, a switching power converter is provided which includes a switch controller and a plurality of transistor switches including a first transistor switch and a group of at least two additional transistor switches. Each transistor switch is coupled to the switch controller for transferring power from a supply to a load by the switch controller opening and closing the corresponding transistor switch. A low power detector is coupled to the group of at least two additional transistor switches for selectively disabling switching of one or more of the transistor switches of the group in accordance with a detected level of power provided to the load.

The plurality of transistor switches may be coupled in parallel to each other. The transistor switches of the plurality that are actively switching may be synchronized. The switch controller may be a pulse-width modulation switch controller. The switch controller may be a frequency modulation switch controller. The low power detector may detect the level of power provided to the load by monitoring a voltage provided to the load or by monitoring an error signal representative of a difference between a voltage provided to the load and a desired level for the voltage. The low power detector may include a comparator having a hysteretic transfer characteristic.

In accordance with another aspect of the invention, a switching amplifier is provided for receiving an input signal and for generating and output signal representative of the input signal. A first pair of transistor switches alternately raises and lowers a voltage potential of a first terminal of a reactive element upon switching of the first pair of transistor switches; a second pair of transistor switches alternately raises and lowers a voltage potential of the first terminal of the reactive element upon switching of the second pair of transistor switches; a switch controller controls switching of the first and second pairs of transistor switches in accordance with the input signal for forming the output signal at a second terminal of the reactive element. A low signal level detector is coupled to the second pair of transistor switches for disabling the second pair of transistor switches from switching upon detection of a low input signal level condition.

The input signal may be an audio signal. The switching amplifier may include a speaker coupled to receive output signal. The switching amplifier may include one or more additional pairs of transistor switches for alternately raising and lowering a voltage potential of the first terminal of the reactive element, each additional pair being selectively disabled by the low signal level detector upon detection of a corresponding low signal level condition. The transistor switches of the first pair may have substantially equal current carrying capacities which are substantially different from current carrying capacities of the transistor switches of the second pair. The low signal level detector may detect the low power condition by monitoring the input signal or by monitoring an error signal representative of a difference between the input signal and a reference level. The low power detector may include a window comparator. The window comparator may have a hysteretic transfer characteristic. The switching amplifier may include a feedback path from a terminal of the reactive element to the switch controller for limiting the output signal. The first pair of transistor switches may be actively switching when the low signal level condition is not detected. The first pair of transistor switches may be disabled by the low signal level detector when the low signal level condition is not detected.

In accordance with yet another aspect of the invention, a switching amplifier is provided for receiving an input signal and for generating a differential output signal representative of the input signal. A first pair of transistor switches alternately raises and lowers a voltage potential of a first terminal of a first reactive element upon switching of the first pair of transistor switches; a second pair of transistor switches alternately raises and lowers a voltage potential of the first terminal of the first reactive element upon switching of the second pair of transistor switches; a third pair of transistor switches alternately raises and lowers a voltage potential of a first terminal of a second reactive element upon switching of the third pair of transistor switches; and a fourth pair of transistor switches alternately raises and lowers a voltage potential of the first terminal of the second reactive element upon switching of the fourth pair of transistor switches. A switch controller controls switching of the first, second, third and fourth pairs of transistor switches in accordance with the input signal for forming a differential output signal across a second terminal of the first reactive element and a second terminal of the second reactive element. A low signal level detector is coupled to the second and fourth pair of transistor switches for disabling the second and fourth pair of transistor switches from switching upon detection of a low input signal level condition.

The input signal may be an audio signal. The switching amplifier may include a speaker coupled to receive the differential output signal. The switching amplifier may include one or more additional pairs of transistor switches for alternately raising and lowering a voltage potential of the first terminal of the reactive element, and one or more additional pairs of transistor switches for alternately raising and lowering a voltage potential of the first terminal of the second reactive element, each additional pair being selectively disabled by the low signal level detector upon detection of a corresponding low signal level condition. The transistor switches of the first pair may have substantially equal current carrying capacities which are substantially different from current carrying capacities of the transistor switches of the second pair. The low signal level detector may be detected the low signal level condition by monitoring the input signal or by monitoring an error signal representative of a difference between the input signal and a reference level. The low power detector may include a window comparator. The window comparator may have a hysteretic transfer characteristic. The switching amplifier may include a feedback path from a terminal of the first reactive element to the switch controller for limiting the differential output signal. A corresponding feedback path from the second reactive element to the switch controller may not be present. The first and third pairs of transistor switches may be actively switching when the low input signal level condition is not detected. The first and third pairs of transistor switches may be disabled by the low signal level detector when the low input signal level condition is not detected. The switching amplifier may be implemented in an eight pin integrated circuit package.

In accordance with a further aspect of the present invention, a switching amplifier is provided for receiving an input signal and for generating a differential output signal representative of the input signal. A first pair of transistor switches alternately raises and lowers a voltage potential of a first terminal of a first reactive element upon switching of the first pair of transistor switches; a second pair of transistor switches alternately raises and lowers a voltage potential of a first terminal of a second reactive element upon switching of the second pair of transistor switches; and a switch controller controls switching of the first and second pairs of transistor switches in accordance with the input signal for forming a differential output signal cross a second terminal of the first reactive element and a second terminal of the second reactive element. A feedback path is provided from a terminal of the first reactive element to the witch controller for limiting the differential output signal. A corresponding feedback path from he second reactive element to the switch controller may not be present.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a switching power converter having a low power mode in accordance with the present invention;

FIGS. 2A-D illustrate timing diagrams for selected signals of the power converter illustrated in FIG. 1;

FIG. 3 illustrates a first alternate embodiment of the power converter of FIG. 1 in which synchronous rectification is performed;

FIG. 4 illustrates a second alternate embodiment of the power converter of FIG. 1 in which parallel paths are provided for charging the output node;

FIGS. 5A-D illustrate timing diagrams for selected signals of the power converter illustrated in FIG. 4;

FIG. 6 illustrates a schematic diagram of a low power detection circuit for the power converter of FIG. 1;

FIG. 7 illustrates a schematic diagram of a switching power amplifier having a low power mode and feedback arrangement;

FIG. 8 illustrates a schematic diagram of a low power detection circuit for the power amplifier of FIG. 7; and

FIG. 9 illustrates a schematic diagram of a class D switching power amplifier having a feedback arrangement and low power mode.

The transistors M₁ and M₂ may function as switches. Activating (closing) one or both of the transistor switches M₁ and M₂ by turning the transistors M₁ and M₂ on results in a current which passes through the inductor L₁. This transfers energy from the supply 102 to an electromagnetic field associated with the inductor L₁. When the transistors M₁ and M₂ are transferred to the capacitor C₁. By repeatedly opening and closing one or both of the transistor switches M₁ and M₂, an output voltage V_(OUT) is built up across the capacitor C₁. The output voltage V_(OUT) is available for providing power to the load 104.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates a schematic diagram of a switching power converter 100 having a low power mode in accordance with the present invention. The power converter 100 is coupled to receive power from a power supply 102. The supply 102 may be, for example, an unregulated direct-current (DC) supply, such as a battery or a rectified alternating current (AC) signal. The supply 102 forms a voltage V_(IN) which may be coupled to a first terminal of a reactive element, such as an inductor L₁. A second terminal of the inductor L₁ may be coupled to a drain terminal of an n-type field-effect transistor M₁, to a drain terminal of an n-type field-effect transistor M₂ and to an anode of a diode D₁. A cathode of the diode D₁ may be coupled to a first terminal of a reactive element, such as a capacitor C₁, and to a first terminal of a load 104. Source terminals of the transistors M₁ and M₂, a second terminal of the capacitor C₁ and a second terminal of the load 104 may be coupled to a ground node. Because the drain and source terminals of the transistors M₁ and M₂ are coupled together, the transistors M₁ and M₂ may be said to be coupled in parallel.

The transistors M₁ and M₂ may function as switches. Activating (closing) one or both of the transistor switches M₁ and M₂ by turning the transistors M₁ and M₂ on results in a current which passes through the indictor L₁. This transfers energy from the supply 102 to an electromagnetic field associated with the inductor L₁. When the transistors M₁ and M₂ are deactivated (opened) by turning the transistors M₁ and M₂ off, energy stored by the inductor L₁ is transferred to the capacitor C₁. By repeatedly opening and closing one or both of the transistor switches M₁ and M₂, an output voltage V_(OUT) is built up across the capacitor C₁. The output voltage V_(OUT) is available for providing power to the load 104.

A switch controller 106 may control the transistor switches M₁ and M₂ based upon a level of the output voltage V_(OUT). The switch controller 106 may include an amplifier 108 and a comparator 110. An inverting input of the amplifier 108 may be coupled to receive the output voltage V_(OUT). A resistive voltage divider (not shown) may be coupled to the inverting input of the amplifier 108 for stepping down the output voltage V_(OUT). A reference voltage V_(REF1) may be coupled to a non-inverting input of the amplifier 108. The reference voltage V_(REF1) corresponds to a desired level for the output voltage V_(OUT). Accordingly, the output of the amplifier 108 forms an error signal V_(EA1) which is representative of a difference between the output voltage V_(OUT) (or a portion of V_(OUT) from the resistive voltage divider) and a desired level for the output voltage V_(OUT). This error signal V_(EA1) may be coupled to a non-inverting input of the comparator 110 while a periodic ramp signal V_(RAMP1) may be coupled to an inverting input of the comparator 110. The output of the comparator 110 forms a switch control signal V_(SW1) which may be coupled to the gate of the transistor switch M₁ for controlling the transistor switch M₁.

FIG. 2A illustrates a timing diagram for the signals V_(EA1) and V_(RAMP1). As shown in FIG. 2A, the error signal V_(EA1) varies in response to changes the output voltage V_(OUT). While the output voltage V_(OUT) is regulated to a desired level, it may undergo transient variations. For example, the output voltage V_(OUT) may vary in response to changes in a level of current drawn by the load 104. As is also shown in FIG. 2A, the ramp signal V_(RAMP1) has a approximately constant slope over each of it periods. The ramp signal V_(RAMP1) is reset to an initial value at the end of each period. The ramp signal V_(RAMP1) may be generated by conventional circuitry.

FIG. 2B illustrates a timing diagram for the switch control signal V_(SW1). Note that FIG. 2B is drawn with a time scale along the horizontal axis which is equivalent to that of FIG. 2A. As shown in FIG. 2B, the switch control signal V_(SW1) may be a logical high voltage, causing the transistor switch M₁ to close, at times when the ramp signal V_(RAMP) is lower than the error signal V_(EA1). As is also shown in FIG. 2B, the signal V_(SW1) may be a logical high voltage, causing the transistor switch M₁ to open, at times when the ramp signal V_(RAMP) is higher than the error signal V_(EA1). Accordingly, as shown in FIG. 2B, a duty-cycle for the transistor switch M₁ may vary with changes in the error signal V_(EA1) so as to regulate V_(OUT) at the desired level.

It will be apparent that the switch controller 106 of FIG. 1 is a conventional PWM switch controller. Because the closing of the transistor switch M₁ coincides with the resetting of the ramp signal V_(RAMP1), the switch controller 106 may be considered to perform trailing edge modulation. It will be apparent that the switch controller 106 may be altered or replaced by another type of switch controller. For example, the switch controller 106 may be modified to perform leading edge modulation. This may be accomplished, for example, by reversing the polarities of the inputs to the amplifier 108 and the comparator 110. As another example, the switch controller 106 may be replaced by a frequency modulation switch controller.

Referring FIG. 1, a low power detector 112 may also be coupled to monitor the error signal V_(EA1). Alternately, the low power detector 112 may be coupled to monitor the output voltage V_(OUT). This is shown in FIG. 1 by a dotted line between the input of the amplifier 108 and the input of the low power detector 112. Assuming the detector 112 is coupled to the input of the amplifier 108, then the detector 112 need not be coupled to the output of the amplifier 108. An output of the low power detector 112 forms a signal V_(LP1) which may be coupled to a first input of a logic AND gate 114. The switch control signal V_(SW1) may be coupled to a second input of the logic AND gate 114. An output of the logic AND gate 114 forms a switch control signal V_(SW2) which may be coupled to the gate of the transistor switch M₂ for controlling the transistor switch M₂. FIG. 2C illustrates a timing diagram for the switch control signal V_(SW2), while FIG. 2D illustrates a timing diagram for the low power detection signal V_(LP1) formed by the low power detector 112. Note that FIGS. 2C-D are drawn with a time scale which coincides with that of FIGS. 2A-B. When the signal V_(LP1) is logic high voltage, the switch control signal V_(SW1) is passed by the logic AND gate 114. Under such circumstances, the transistor switch M₂ may be synchronized with the transistor switch M₁ and may operate in accordance with the same duty cycle, as shown by comparing FIGS. 2B and 2C prior to a time T₁ and after a time T₄.

However, when the low power detector 112 detects that the load 104 draws a relatively light load, the low power detection signal V_(LP1) may transition from a logic high voltage to a logic low voltage. This is shown occurring in FIG. 2D sometime after the time T₁ and before the time T₂. In response to this transition in the signal V_(LP1), the logic AND gate 114 inhibits the switch control signal V_(SW1) from passing to its output. Instead, the signal V_(SW2) formed at the output of the logic AND gate 114 is held at a logic low voltage. This is shown in FIG. 2C at the time T₂ where the signal V_(SW2) remains a logic low voltage, whereas, the signal V_(SW1) transitions from a logic low voltage to a logic high voltage. Accordingly, the transistor switch M₂ is disabled upon detection of a low power condition. Assuming a low power condition is no longer detected, then the signal V_(LP1) may transition back to a logical high voltage, as shown occurring in FIG. 2D after a time T₃ and before the time T₄. As a result, the transistor switch M₂ is enabled to resume switching in concert with the transistor switch M₁, as shown occurring in FIG. 2D at the time T₄.

This feature of the present invention may provide significant efficiency benefits when the power requirements of the load 104 change significantly over time. For example, the load 104 may be a battery-powered device, such as a portable telephone. When the telephone is in use, such as for making a telephone call, then its receiver and transmitter may both be active. Under these circumstances, both the transistor switches M₁ and M₂ may be actively switching to supply sufficient power to the telephone. When the call ends, the telephone may enter a standby mode in which the receiver is active, but the transmitter is inactive. As a result, less power is drawn by the telephone. This may cause the output voltage V_(OUT) supplied to the telephone to increase somewhat and, thus, the level of error signal V_(EA1) may fall. Note that when the load 104 draws a low level of power, the output voltage V_(OUT) tends to rise due to opening and closing of the switches M₁ and M₂. This is because even at a minimum duty-cycle, switching may still occur. As a result, the low power detector 112 may detect that the telephone is in a low power mode and, in response, disables switching of the transistor switch M₂. Switching of the switch M₁, by itself, may then provide sufficient power for the portable telephone when it is in standby mode. When the portable telephone is again used for a telephone call, then its transmitter may be activated. This may cause the output voltage V_(OUT) to fall somewhat and, thus, the error signal V_(EA1) may rise. As a result, the low power detector 104 may detect that the telephone is in a high power mode and, in response, re-enables switching of the transistor switch M₂.

Switching losses in the transistor switches M₁ and M₂ are largely a result of parasitic capacitance at their gates, This parasitic gate capacitance is charged and then discharged each time the corresponding transistor is activated and then deactivated. The gate capacitance of a transistor is related to the size of the current-carrying channel of the transistor. For a small transistor which has a small current-carrying capacity, the gate capacitance tends to be relatively small. In contrast, for a large transistor which has a large current-carrying capacity, the gate capacitance tends to be relatively large. Because the transistor switch M₂ is disabled during periods of low power consumption, switching losses associated with the switch M₂ are essentially eliminated during such periods. While switching losses are associated with the transistor M₁ during low power mode operation, these switching losses may less than would be the case for a single transistor which had current capacity for worst-case power requirements of the load 104. In accordance with the present invention, the transistor switch M₂ is brought into play only as necessary, such as when needed to satisfy the power requirements of the load 104.

In the preferred embodiment, the transistor switches M₁ and M₂ are sized so as to correspond with the various power requirements of the load 104. Considering the portable telephone example, power consumed in the standby mode can be expected to be less than approximately one-fourth of the power consumed when the telephone is being used during a telephone call. Accordingly, the transistor M₁ may have a substantially smaller current carrying capacity than the transistor M₂. For other loads, the proportions of power consumed in different operating modes may differ. For example, assume a load operates one of two modes where the power consumed in a first mode is approximately eighty percent of the power consumed in a second mode. For such a load, the transistor M₁ may have a substantially greater current carrying capacity than the transistor M₂. For other loads, the transistors M₁ and M₂ may have substantially equal current-carrying capacities. In addition to loads which draw power at relatively discrete levels, the invention may also provide efficiency advantages for loads which draw continuously varying levels of power.

Further, in addition to providing a single transistor which may be selectively disabled, such as the transistor M₂ (FIG. 1), it will be apparent that one or more transistors may be provided, one or more of which may be selectively disabled depending upon the level of power consumed by the load. For example, for a load which operates primarily in three distinct modes, each having a corresponding level of power consumption, a power converter may be provided for powering the load with three transistors switches having appropriate current-carrying capacity, one or two of which are selectively disabled depending upon the level of power consumed by the load. The low power detector 112 may then provide multiple outputs to specify one of a plurality of such low power modes. It will be apparent also that by appropriately modifying the logic circuitry of FIG. 1, the transistor M₁ may be disabled from switching when the transistor M₂ is actively switching.

The diode D₁ of FIG. 1 performs signal rectification such that current flows from the inductor L₁ for charging the capacitor C₁, however, current is prevented by the diode D₁ from discharging the capacitor C₁ through the transistor switches M₁ and M₂. FIG. 3 illustrates an alternate embodiment of the power converter 100 of FIG. 1 in which synchronous rectification is performed. More particularly, the diode D₁ of FIG. 1 is replaced by n-type transistors M₃ and M₄. The transistor M₃ receives the switch control signal V_(SW1), while the transistor M₄ receives a switch control signal V_(SW3). The switch control signal V_(SW1) is coupled to a first input of an inverter 116. An output of the inverter 116 is coupled to a first input of a logic AND gate 118. A second input of the logic AND gate 118 is coupled to receive the signal V_(LP1) from the low power detector 112. An output of the logic AND gate 118 is coupled to an input of an inverter 120. An output of the inverter 120 forms the switch control signal V_(SW3) which is coupled to the gate of the transistor M₄.

Under normal operating conditions, the transistor switch M₃ is open when the transistor switch M₁ is closed and vice versa. Similarly, the transistor switch M₄ is open when the transistor switch M₂ is closed and vice versa. Accordingly, during normal operation, the transistor switches M₃ and M₄ operate synchronously with the transistor switches M₁ and M₂ to rectify the current flowing to the capacitor C₁ from the inductor L₁. It will be understood, that the controller 106′ preferably includes make-before-break circuitry (not shown) which ensures that the transistors M₁ and M₃ are not both turned on at the same time. Similarly, the controller 106′ preferably ensures that the transistor switches M₂ and M₄ are not both turned on at the same time.

In the low power mode, when the signal V_(LP1) is logic low voltage, the transistor switches M₂ and M₄ are disabled from switching. Under such conditions, each transistor switch M₂ and M₄ is turned off.

The transistor switches M₃ and M₄ are preferably sized substantially the same as the corresponding transistor switches M₁ and M₂, respectively. Synchronous rectification may also be accomplished by omitting the transistor M₂ and sizing the transistor M₁ appropriately to carry all of the current from the inductor L₁ to the capacitor C₁.

FIG. 4 illustrates a second alternate embodiment of the power converter of FIG. 1 in which parallel paths are provided for delivering power to the load 104. The power converter 100″ of FIG. 4 differs from that of FIG. 1 in that a second inductor L₂ provides a second path through which the capacitor C₁ may be charged from the input supply V_(IN). Current through the second inductor L₂ is controlled by the transistor switch M₂. Accordingly, the input supply V_(IN) is coupled to a first terminal of the inductor L₂. A second terminal of the inductor L₂ is coupled to the drain of the transistor switch M₂ and to an anode of a diode D₂. A cathode of the diode D₂ is coupled the first terminal of the capacitor C₁.

Under normal operating conditions, the transistor switches M₁ and M₂ (FIG. 4) are both switching so that each provides a portion of the power delivered to the load 104. In the preferred embodiment, the transistor switches M₁ and M₂ are operated in an interleaved fashion. Thus, the switch M₁ is cycled from closed-to-open-to-closed while the switch M₂ remains closed. Similarly, the switch M₂ is cycled from closed-to-open-to-closed while the switch M₁ remains closed. FIGS. 5A-C illustrate timing diagrams for selected signals of the power converter illustrated in FIG. 4. The transistor switches M₁ and M₂ of FIG. 4 are controlled by switch control signals V_(SW1)′ and V_(SW2)′, respectively.

As can be seen from FIGS. 5A-SD, when the output V_(LP1) of the low power detector 112 is a logic high voltage, the transistor switches M₁ and M₂ are both actively switching in an interleaved fashion. When the low power detector 112 detects a low power condition, however, the signal V_(LP1) transitions to a logic low voltage. As shown in FIGS. 5C-5D, this disables the transistor switch M₂ from switching until the signal V_(LP1) returns to a logic high voltage.

The switch controller 106′ of FIG. 4 differs from that of FIG. 1 in that a flip-flop 122 and logic AND gates 124 and 126 provide the switch control signals V_(SW1)′ and V_(SW2)′. More particularly, a clock signal V_(CLK) may be coupled to a toggle input of the flip-flop 122. Preferably, the flip-flop 122 is leading edge triggered while leading edges of the clock signal V_(CLK) correspond with resetting of the ramp signal V_(RAMP1), as shown in FIG. 5A. Thus, a Q output of the flip-flop 122 provides square wave signal of one-half the frequency of the clock signal V_(CLK) and is coupled to a first input of the logic AND gate 126. An inverted output {overscore (Q)} of the flip-flop 122 is coupled to a first input of the AND gate 124. A second input of each AND gate 124 and 126 is coupled to receive an output signal V_(CMP) from the comparator 110. The logic AND gate 124 provides the switch control signal V_(SW1)′ for controlling the switch M₁. Similarly, the logic AND gate 126 provides the switch control signal V_(SW2)′ for controlling the switch M₂.

The inductors L₁ and L₂ may be of the same inductance value or may be of different values. In the preferred embodiment, the inductors L₁ and L₂ have inductance values which correspond to the sizes of the transistor switches M₁ and M₂ and to various levels of power expected to be drawn by the load 104. Further, one or both of the diodes D₁ and D₂ may be replaced by switches for performing synchronous rectification.

FIG. 6 illustrates a schematic diagram of the low power detector 112 of FIG. 1. A reference voltage V_(REF2) may be coupled to a first terminal of a resistor R₁. A second terminal of the resistor R₁ may be coupled to a first terminal of a resistor R₂ and to a first terminal of a comparator 128. A second terminal of the resistor R₂ may be coupled to the ground node. The resistors R₁ and R₂ form a resistive divider. Accordingly, a reference voltage V_(REF3) formed at the first input of the comparator 128 is proportional to the reference voltage V_(REF2). As mentioned, the low power detector 112 may be coupled to monitor either the output voltage V_(OUT) or the error signal V_(EA1) to determine whether a load coupled to the power converter 100 is in a low power mode. Assuming the error signal V_(EA1) is monitored, it may be coupled to a second input of the comparator 128. An output of the comparator 128 forms the signal V_(LP1).

As mentioned, under normal operating conditions, when both of the transistor switches M₁ and M₂ are operating, the signal V_(LP1) is a logical high voltage. When the level of V_(EA1) falls below the level of V_(REF3), this indicates that the load 104 is in a low power mode. The comparator 128 responds by changing the signal V_(LP1) from a logical high voltage to a logical low voltage. This disables switching of the transistor M₂. Preferably, the comparator 128 has a hysteretic transfer characteristic. This prevents the transistor M₂ from being disabled and enabled unnecessarily when the signal V_(EA1) hovers about the level of the reference voltage V_(REF3). Thus, the signal V_(LP1) preferably does not return to a logic high voltage until the level of V_(EA1) rises above the level of V_(REF3) by a predetermined amount, as determined by the amount of hysteresis of the comparator 128.

Note that assuming more than two transistor switches are provided, the low power detector 112 may include an additional comparator for each additional transistor switch along with appropriate other logic circuitry. Each additional comparator may have a first input coupled to V_(EA1) (or V_(OUT)), a second input coupled to an appropriate reference voltage level and an output coupled to enable/disable the corresponding transistor switch.

In sum, a switching power converter is disclosed which includes a plurality of main power switches, at least one of which may be disabled from switching while the others continue actively switching when a load powered by the converter draws a low level of power. Optionally, at least one transistor switch utilized for synchronous rectification may also be disabled in response to a low level of power. This feature is advantageous as it reduces switching losses and, thus, increases efficiency for periods of low power consumption by the load. Because the load may operate in a low power mode for an extended period of time, such as standby mode in a battery-powered telephone, the prevention of power loss may contribute considerably to operating time before the battery re-charging is needed.

FIG. 7 illustrates a schematic diagram of a switching power amplifier 200 having a low power mode and a feedback arrangement in accordance with the present invention. Referring to FIG. 7, a supply voltage V_(CC) may be coupled to a source terminal of a p-type field-effect transistor M₅ and to a source terminal of an n-type field-effect transistor M₆. A drain terminal of the transistor M₅ may be coupled to a drain terminal of the transistor M₆, to a drain terminal of an n-type field-effect transistor M₇, to a drain terminal of an n-type field-effect transistor M₈ and to a first terminal of an inductor L₃. A second terminal of the inductor L₃ may be coupled to a first terminal of a capacitor C₂. A source terminal of the transistor M₇, a source terminal of the transistor M₈ and a second terminal of the capacitor C₂ may be coupled to a ground node.

The transistors M₅-M₈ may function as switches. Preferably, the pair of transistor switches M₅ and M₇ are operated such that when the transistor switch M₅ is activated (closed), the transistor switch M₇ is deactivated (opened); and when the transistor switch M₅ is deactivated (opened), the transistor switch M₇ is activated (closed). The pair of transistor switches M₆ and M₈ may be selectively disabled from switching. When disabled, the transistor switches M₆ and M₈ are both deactivated (opened). When actively switching, the transistor switches M₆ and Mg are preferably operated such that the transistor switch M₆ is in the same condition (opened or closed) as the transistor switch M₅ and the transistor switch M₈ is in the same condition as the transistor switch M₇. Thus, when actively switching, the pair of transistor switches M₆ and M₈ are synchronized with the corresponding transistor of the pair M₅ and M₇.

Accordingly, when one or both of the transistor switches M₅ and M₆ is closed, the transistor switches M₇ and M₈ are open. Under these conditions, a voltage at the first terminal of the inductor L₃ is increased As a result, a current from the supply voltage V_(CC) tends to flow through the inductor L₃ to the capacitor C₂. This tends to increase a voltage across the capacitor C₂. When one or both of the transistor switches M₇ and M₈ is closed, the transistor switches M₅ and M₆ are open. Under these conditions, the voltage at the first terminal of the inductor L₃ is decreased. As a result, a current tends to be drawn from the capacitor C₂ and through the inductor L₃ to the ground node. This tends to reduce the voltage across the capacitor C₂. The inductor L₃ and the capacitor C₂ function as a smoothing filter such that a voltage formed across the capacitor C₂ is representative of an average of the voltage at the first terminal of the inductor L₃.

A speaker 202 may be coupled to receive an audio output signal AUD_(OUT) formed across the capacitor C₂. More particularly, a first terminal of an inductor L₄ may be coupled to the first terminal of the capacitor C₂. A second terminal of the inductor L₄ may be coupled to a first terminal of the speaker 202. A second terminal of the speaker 202 may be coupled to a first terminal of a capacitor C₃. A second terminal of the capacitor C₃ may be coupled to the ground node. Alternately, the speaker 202 may be coupled to the capacitor C₂ via a transformer (not shown) In which case, a primary winding of the transformer may be coupled in series between the first terminal of the capacitor C₂ and the ground node, while the speaker may be coupled across a secondary winding of the transformer.

A switch controller 204 may be coupled to control the switches M₅-M₈ for powering a speaker 202 based upon an input audio signal AUD_(IN). More particularly, the audio input signal AUD_(IN) may be coupled to a first terminal of a resistor R₃. A second terminal of the resistor R₃ may be coupled to the switch controller 204. The switch controller 204 may include an amplifier 206, a comparator 208 and a compensation capacitor C₄. An inverting input of the amplifier 206 may be coupled to the second terminal of the resistor R₃ and to a first terminal of the capacitor C₄. A reference voltage V_(REF4) may be coupled to a non-inverting input of the amplifier 206. The reference voltage V_(REF4) may be set to a desired quiescent level for the audio output signal AUD_(OUT) (e.g., V_(CC)/2). Thus, the switch controller 204 receives the audio input signal AUD_(IN) via the resistor R₃ and forms the error signal V_(EA2) at the output of the amplifier 206. The error signal V_(EA2) is representative of a deviation in the AUD_(IN) signal from the reference voltage V_(REF4).

The output of the amplifier 206 may be coupled to a second terminal of the capacitor C₄ and to a non-inverting input of the comparator 208. An inverting input of the comparator 208 may be coupled to receive a periodic ramp signal V_(RAMP2), similar to the ramp signal V_(RAMP1) (FIG. 2A). The error signal V_(EA2) is then compared by the comparator 208 to the ramp signal V_(RAMP2). An output of the comparator 208 forms a switch control signal V_(SW4) which may be coupled to a gate of the transistor switches M₃ and M₇ for controlling the transistor switches M₅ and M₇.

Accordingly, the switch control signal V_(SW4), formed by the comparator 208, is pulse-width modulated in accordance with the audio input signal AUD_(IN) so as to control the transistors M₅ and M₇ to form the audio output signal AUD_(OUT) at the first terminal of the capacitor C₂. A resistor R₄ may be coupled to provide negative feedback, such as from the second terminal of the inductor L₂ to the second terminal of the resistor R₃.

A low power detector 210 may be coupled to monitor the error signal V_(EA2). Alternately, the low power detector 210 may be coupled to monitor the audio input signal AUD_(IN), as shown by the dotted line between the second terminal of the resistor R₃ and the input of the detector 212. An output V_(LP2) of the low power detector 210 may be coupled to a first input terminal of a logic AND gate 212 and to a first input terminal of a logic AND gate 214. The switch control signal V_(SW4) may be coupled to a second input of the logic AND gate 214 and to a second input of the logic AND gate 212 via an inverter 216. An output of the logic AND gate 212 forms a switch control signal V_(SW5) which may be coupled to the gate of the transistor switch M₆ via an inverter 218. An output of the logic AND gate 214 forms a switch control signal V_(SW6) which may be coupled to the gate of the transistor switch M₈.

Thus, when the low power detection signal V_(LP2) is a logical high voltage, the switch control signal V_(SW4) is passed by the logic AND gate 214 to form the switch control signal V_(SW6) and the switch control signal V_(SW4), as inverted by the inverter 216, is passed by the logic AND gate 212 to form the switch control signal V_(SW5). Under such conditions, the transistor switches M₆ and M₈ may actively switch in synchronism with the switches M₅ and M₇, respectively. However, when the input signal AUD_(IN) is relatively low in amplitude, this means that the speaker 202 requires less power than when the signal AUD_(IN) is of relatively high amplitude. Accordingly, the low power detector 210 detects this condition by monitoring the error signal V_(EA2) (or the input signal AUD_(IN)). In response, the output V_(LP2) of the low power detector 210 may transition from a logic high level to a logic low level. When the signal V_(LP2) is a logic low voltage, the switch control signals V_(SW5) and V_(SW6) are held to a logic low level by the AND gates 212 and 214, respectively. Under these conditions, the transistor switches M₆ and M₈ are disabled from switching.

In summary, under low audio input signal AUD_(IN) levels, such a low volume setting, less power is required to drive the speaker 202 than would be required otherwise. Accordingly, to save power, the transistors M₆ and M₈ are disabled from switching. This feature provides efficiency benefits by avoiding switching losses caused by the transistor switches M₆ and M₈.

FIG. 8 illustrates a schematic diagram of the low power detector 210 of FIG. 7. Referring to FIG. 8, a reference voltage V_(REF5) may be coupled to a first terminal of a resistor R₅. A reference voltage V_(REF6) is formed at a second terminal of the resistor R₅ and may be coupled to a first terminal of a resistor R₆ and to a first terminal of a comparator 220. A reference voltage V_(REF7) is formed at a second terminal of the resistor R₆ and may be coupled to a first terminal of a resistor R₇ and to a first terminal of a comparator 222. A second terminal of the resistor R₇ may be coupled to the ground node. The error signal V_(EA2) may be coupled to a second input of the comparator 220 and to a second input of the comparator 222. An output of the comparator 220 may be coupled to a first input of a logic OR gate 224, while an output of the comparator 222 may be coupled to a second input of the logic OR gate 224. An output of the logic OR gate 224 forms the signal V_(LP2).

Together, the comparators 220 and 222 and the logic OR gate 224 form a window comparator such that the output V_(LP2) is a logic high voltage, and the transistors M₆ and M₈ (FIG. 7) are enabled, only when its input signal V_(EA2) is higher than V_(REF6) or lower than V_(REF7). When the error signal V_(EA2) is between the levels of V_(REF6) and V_(REF7), then a low power condition is indicated in which the output signal V_(LP2) is a logic low voltage and the transistor switches M₆ and M₈ are disabled from switching. The levels of V_(REF6) and V_(REF7) can be selected to adjust the thresholds at which the low power detector 210 indicates a low power condition. A window comparison is preferred over a single threshold comparison because the audio input signal AUD_(IN) is expected to be an alternating current (AC) signal with positive and negative swings. Thus, the upper threshold V_(REF6) of the window comparator detects positive swings in the audio input signal AUD_(IN) and the lower threshold V_(REF7) detects negative swings. In the preferred embodiment, the comparators 220 and 222 each have a hysteretic transfer characteristic so as to avoid enabling and disabling the transistor switches M₆ and M₈ excessively should the input signal V_(EA2) hover about one of the threshold voltages V_(REF6) or V_(REF7).

It will be apparent that appropriate modifications to the logic circuitry of FIG. 7 may be made to disable transistor pair M₅ and M₆ from switching, if desired, when the transistor pair M₆ and M₈ are actively switching. Thus, only one of the pairs of transistor switches may be actively switching at a time. It will also be apparent that the power amplifier 200 may be provided with additional pairs of transistor switches which may be selectively enabled and disabled according to the level of the input signal AUD_(IN) or the error signal V_(EA2).

FIG. 9 illustrates a schematic diagram of a class D switching power amplifier 300 having a feedback arrangement and low power mode in accordance with the present invention. The power amplifier 300 of FIG. 9 is similar to the power amplifier 200 illustrated in FIG. 7 with important differences discussed below. Elements of FIG. 9 having a one-to-one functional correspondence with those of FIG. 7 are given the same reference numeral. An important difference between the amplifier 200 (FIG. 7) and the amplifier 300 (FIG. 9) is that the amplifier 300 includes a second switching stage 304 in addition to a first switching stage 302. Elements of the second switching stage 304 having a one-to-one functional correspondence with elements of the first switching stage 302 are given the same reference numerals primed.

The pair of transistor switches M₆′ and M₈′ of the second switching stage 304 may be disabled from switching when the transistor switch pair M₆ and M₈ of the first stage 302 are disabled from switching. This is apparent from FIG. 9 because when the low power detection signal V_(LP2) is logic low voltage, the signal levels applied to the gates of the transistors M₆′ and M₈′ will cause these transistor switches to be off (open). For example, the switch control signal V_(SW4) is a logic low voltage and is applied to the transistor M₈. In addition, the switch control signal V_(SW5) is a logic low voltage which is inverted by the inverter 306 and applied to the transistor M₆′. However, when low power detection signal V_(LP2) is a logic high voltage, the transistor switches M₆′ and M₈′ are actively switching, the transistor M₆′ is in the same condition (opened or closed) as the transistor M₈. This is apparent from FIG. 9. Similarly, when actively switching, the transistor M₈′ is in the same condition (opened or closed) as the transistor M₆. This is also apparent from FIG. 9. In addition, it can be seen from FIG. 9 that the transistor switches M₅ and M₇ receive the switch control signal V_(SW4), while the transistor switches M₅′ and M₇′ receive the switch control signal V_(SW4), after being inverted by inverter 306.

Thus, by providing the second switching stage 304, the switching power amplifier 300 includes an H-bridge arrangement of transistor switches for driving a load, such as the speaker 202, with a differential output signal. This is in contrast to the switching power amplifier 200 of FIG. 7 in which the speaker 202 is driven with a single-ended output signal.

In contrast to a conventional class D audio amplifier, the amplifier 300 illustrated in FIG. 9 provides a negative feedback signal from the first switching stage 302 to the switch controller 204 via the resistor R₄. More particularly, the resistor R₄ may be coupled between the first terminal of the resistor R₃ and the second terminal of the inductor L₃. Although feedback is preferably not provided from the second switching stage 304, the output of the second stage 304 is expected to be limited by the feedback signal from the first stage 302 since the two stages 302 and 304 are expected to have comparable behavior. This is because the transistor switches M₅-M₈ and M₅′-M₈′ may be formed monolithically (i.e., the transistor switches M₅-M₈ and M₅′-M₈′ may all be formed on one integrated circuit chip). Accordingly, the amplifier 300 of FIG. 7 provides a differential output audio signal across the speaker 202 which is controlled with feedback to closely follow the input audio signal AUD_(IN). It will be apparent that this feedback feature of the present invention may be implemented in a switching amplifier, as described in reference to FIG. 9, even assuming the selectively disabled transistor switches M₆, M₆′, M₈ and M₈′ are omitted. By providing such feedback, the amplifier 300 is less susceptible to distortion than a conventional class D amplifier operated in open loop.

In addition, the amplifier 300 of FIG. 9 provides a low power mode at times when the input audio signal AUD_(IN) is of relatively low amplitude. More particularly, when the low power detector 210 of FIG. 9 detects that the audio input signal AUD_(IN) is at a relatively low level, the low power detector 210 provides a logic low voltage to the logic AND gates 212 and 214. In response, the transistor switches M₆, M₆′, M₈, and M₈′ are disabled from switching. Under these conditions, only the transistors M₅, M₅′, M₇ and M₇′ may be actively switching so as to provide power to the speaker 202. Alternately, with appropriate modifications to the logic circuitry of FIG. 7, the transistor switches M₅, M₅′, M₇ and M₇′ may be disabled from switching when the transistor switches M₆, M₆′, M₈ and M₈′ are actively switching.

Further, the amplifier 300 of FIG. 9 may be implemented as an integrated circuit chip package having eight pins, as shown in FIG. 9. A pin (1) may be coupled to a node between the resistors R₃ and R₄; a second pin (2) may be coupled as an output of a power supply 310 for providing a regulated voltage V_(CC) to circuitry of the amplifier 300; a third pin (3) may be coupled as an input to the power supply 310 for selectively shutting down the amplifier 300; a fourth pin (4) may be coupled as in input to the power supply 310 for providing an unregulated voltage to the power supply; a fifth pin (5) may be coupled to an output of the first switching stage 302 (e.g., to a first terminal of the inductor L₃); a sixth pin (6) may be coupled to an output of the second switch stage 304 (e.g., to a first terminal of the inductor L₃′); a seventh pin (7) may be coupled to a ground node of the amplifier 300; and an eighth pin (8) may be coupled to provide the ramp signal V_(RAMP2) to the switch controller 204.

While the foregoing has been with reference to particular embodiments of the invention, it will be appreciated by those skilled in the art that changes in these embodiments may be made without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims. For example, it will be apparent that the equivalent logic functions can be implemented using circuitry other than that illustrated herein. Further, it will be apparent that signals which are disclosed herein as active high can be made active low and that signals that are disclosed as active low can be made active high. Similarly, gate drive signals for the various transistors may be formed by additional circuitry, such as buffers or charge pumps. Further, the transistors disclosed herein may be replaced by another type of transistor with appropriate modifications. Accordingly, n-type and p-type transistors may be exchanged for the other or for a npn or pnp bipolar transistor. 

What is claimed is:
 1. A switching power converter comprising: a first transistor switch for transferring energy from a supply to a load by opening and closing the first transistor switch; a second transistor switch coupled in parallel with the first transistor switch for transferring energy from the supply to the load by opening and closing the second transistor switch; and a low power detector coupled to the second transistor switch for disabling switching of the second transistor switch upon detection of a low power condition.
 2. The switching power converter according to claim 1, wherein switching of the first transistor switch is synchronized with switching of the second transistor switch when the second transistor switch is actively switching.
 3. The switching power converter according to claim 1, wherein the first transistor switch is disabled from switching by the low power detector when the second transistor switch is actively switching.
 4. The switching power converter according to claim 1, further comprising a pulse-width modulation switch controller coupled to the first and second switches.
 5. The switching power converter according to claim 1, further comprising a frequency modulation switch controller coupled to the first and second switches.
 6. The switching power converter according to claim 1, wherein the low power detector detects the low power condition by monitoring a voltage provided to the load.
 7. The switching power converter according to claim 1, wherein the low power detector detects the low power condition by monitoring an error signal representative of a difference between a voltage provided to the load and a desired level for the voltage.
 8. The switching power converter according to claim 1, wherein the low power detector comprises a comparator having a hysteretic transfer characteristic.
 9. The switching power converter according to claim 1, wherein the first and second transistor switches have substantially different current-carrying capacities.
 10. The switching power converter according to claim 1, wherein the energy is transferred to the load by a current which is synchronously rectified.
 11. The switching power converter according to claim 10, further comprising a third transistor switch and a fourth transistor switch, the third and fourth transistor switches for performing synchronous rectification on the current, wherein the fourth transistor is disabled by the low power detector upon detection of the low power condition.
 12. A switching power converter comprising: a first transistor switch for transferring energy from a supply to a load by opening and closing the first transistor switch; a second transistor switch coupled to the first transistor switch for transferring energy from the supply to the load by opening and closing the second transistor switch, wherein when the second transistor switch is actively switching, the second transistor switch is closed when the first transistor switch is closed and the second transistor switch is open when the first transistor switch is open; and a low power detector coupled to the second transistor switch for disabling switching of the second transistor switch upon detection of a low power condition.
 13. The switching power converter according to claim 12, wherein the first transistor switch is disabled from switching by the low power detector when the second transistor switch is actively switching.
 14. The switching power converter according to claim 12, further comprising a pulse-width modulation switch controller coupled to the first and second switches.
 15. The switching power converter according to claim 12, further comprising a frequency modulation switch controller coupled to the first and second switches.
 16. The switching power converter according to claim 12, wherein the low power detector detects the low power condition by monitoring a voltage provided to the load.
 17. The switching power converter according to claim 12, wherein the low power detector detects the low power condition by monitoring an error signal representative of a difference between a voltage provided to the load and a desired level for the voltage.
 18. The switching power converter according to claim 12, wherein the low power detector comprises a comparator having a hysteretic transfer characteristic.
 19. The switching power converter according to claim 12, wherein the first and second transistor switches have substantially different current-carrying capacities.
 20. The switching power converter according to claim 12, wherein the energy is transferred to the load by a current which is synchronously rectified.
 21. The switching power converter according to claim 20, further comprising a third transistor switch and a fourth transistor switch, the third and fourth transistor switches for performing synchronous rectification on the current, wherein the fourth transistor is disabled by the low power detector upon detection of the low power condition.
 22. A switching power converter comprising: a switch controller; a plurality of transistor switches coupled in parallel to each other and including a first transistor switch and a group of at least two additional transistor switches, each transistor switch coupled to the switch controller for transferring power from a supply to a load by the switch controller opening and closing the corresponding transistor switch; and a low power detector coupled to the group of at least two additional transistor switches for selectively disabling switching of one or more of the transistor switches of the group in accordance with a detected level of power provided to the load.
 23. The switching power converter according to claim 22, wherein the transistor switches of the plurality that are actively switching are synchronized.
 24. The switching power converter according to claim 22, wherein the switch controller is a pulse-width modulation switch controller.
 25. The switching power converter according to claim 22, wherein the switch controller is a frequency modulation switch controller.
 26. The switching power converter according to claim 22, wherein the low power detector detects the level of power provided to the load by monitoring a voltage provided to the load.
 27. The switching power converter according to claim 26, wherein the low power detector comprises a comparator having a hysteretic transfer characteristic.
 28. The switching power converter according to claim 22, wherein the low power detector detects the level of power provided to the load by monitoring an error signal representative of a difference between a voltage provided to the load and a desired level for the voltage.
 29. The switching power converter according to claim 28, wherein the low power detector comprises a comparator having a hysteretic transfer characteristic.
 30. A switching power converter comprising: a switch controller; a plurality of transistor switches including a first transistor switch and a group of at least two additional transistor switches, each transistor switch coupled to the switch controller for transferring power from a supply to a load by the switch controller opening and closing the corresponding transistor switch, wherein switches of the group that are actively switching are closed when the first transistor is closed and open when the first transistor switch is open; and a low power detector coupled to the group of at least two additional transistor switches for selectively disabling switching of one or more of the transistor switches of the group in accordance with a detected level of power provided to the load.
 31. The switching power converter according to claim 30, wherein the plurality of transistor switches are coupled in parallel to each other.
 32. The switching power converter according to claim 30, wherein the switch controller is a pulse-width modulation switch controller.
 33. The switching power converter according to claim 30, wherein the switch controller is a frequency modulation switch controller.
 34. The switching power converter according to claim 30, wherein the low power detector detects the level of power provided to the load by monitoring a voltage provided to the load.
 35. The switching power converter according to claim 34, wherein the low power detector comprises a comparator having a hysteretic transfer characteristic.
 36. The switching power converter according to claim 30, wherein the low power detector detects the level of power provided to the load by monitoring an error signal representative of a difference between a voltage provided to the load and a desired level for the voltage.
 37. The switching power converter according to claim 36, wherein the low power detector comprises a comparator having a hysteretic transfer characteristic. 